With the burgeoning of electronic technologies, the demand for a wider variety of communications services has been steadily rising. New technologies have been continually developing to meet these demands including lasers, fiber optics, microprocessors, and very large scale integrated circuits. While many alternative approaches for realizing the new communications services are available, digital data transmission seems to be the one best equipped for future progress and, accordingly, it is currently receiving the most attention.
In terms of the services to be offered, it is expected that, in addition to standard telephone services, a host of low, medium and wide bandwidth services would be introduced. This would include electronic mail, facsimile, high-fidelity audio, computerized data base searches (e.g., want ads, encyclopedia, travel arrangements, etc.), remote shopping, electronic banking and home computer networks. Business users would also use electronic blackboards, teleconference facilities, word processors, and large computer communication networks.
To realize a communications system that can efficiently support such a rich menu of services one must consider the nature of the signals flowing through the system and the demands that these signals place thereon.
It is clear, for example, that at least for some of the services wide bandwidth capability is required of the transmission medium and of the switching equipment.
It is also clear that the signals of such services are of diverse nature. For example, some of the services are characterized by bursty signals (e.g., computer data streams) while others are characterized by continuous signals (e.g., video). Whereas delays or interruptions in bursty signals are often acceptable, substantial delays of continuous signals are unacceptable and interruptions are very difficult to patch. Accordingly, different hardware capabilities are suitable for bursty and continuous signals.
Different signal interactions between users also call for different hardware capabilities. Conventional telephone conversations are one-to-one communications, requiring low bandwidth and two way transmission. Services that provide information, such as time and weather, are one-to-many communications, requiring broadcast capability but only one way transmission. A third mode of communications, many-to-many, results when a user subscribes to a number of services and simultaneously employs those services to communicate with several other users (such as listening on the telephone while receiving electronic mail). These different modes of communication require different capabilities from a communications system.
Closely tied to the above considerations is the per user capacity that is made available. The effects of per user capacity considerations manifest themselves both in the switch and the transmission channels interactions.
With respect to the switch, since some of the new services will require a wide bandwidth, the switch will have to possess wide bandwidth capabilities. However, most switches that offer wide bandwidth capability underutilize this bandwidth when signals of low bandwidth are transmitted. That represents a wasted resource. To minimize waste with the myriad of services that are expected to be offered, the switch employed has to inherently be capable of wide bandwidth switching, and yet offer to the users only the bandwidth they need (or wish to pay for) so that bandwidth unused by one user can be offered to another user.
As for the transmission channel interactions, one of the more economical methods of connecting the switch to customers who send and receive a number of services (many-to-many) is via single cables having the several signals multiplexed thereon. One of the main problems with this approach, however, is the difficulty in coordinating the transmissions between two or more sources and a common receiver. Conventionally, the receiver's capacity is partitioned into time slots which can be allocated among the transmitters but, unfortunately, this situation is blocking. A transmitter might not be able to communicate with a particular receiver because it might have previously committed to another receiver the time slots which this particular receiver has available. So, while the receiver and the transmitter might have the necessary bandwidth, communication would not be possible.
In summary, it is desirable to have a switch that is capable of switching signals in a wide range of bandwidths, of economically utilizing its wideband resources to switch lower bandwidth signals, handling bursty as well as continuous signals, and of efficiently establishing one-to-one, one-to-many, and many-to-many modes of communication without blocking.